Electric motors benefit from frequency control technology 1

The speed of an induction motor depends primarily on the number of pole pairs (windings in the stator) and the frequency of the supply voltage. The amplitude of the voltage and the load on the motor shaft also influence the speed but to a lesser extent. As constantly varying the pole pairs is not an option, altering the frequency of the supply voltage is the best method for controlling the motor speed.

AC drives are devices that use frequency control to vary motor speed. The main benefit of controlling the speed of a cargo pump is to optimise performance when discharging cargos of varying density and viscosity. Other benefits include providing a ‘soft’ start and stop that reduces the wear of mechanical parts, and being able to adjust the motor speed in response to demand increases efficiency and saves energy.

The speed is controlled by changing the frequency of the electrical supply to the motor. The 3-phase voltage supplied by the onboard generator is connected to a standard induction motor creating a rotating magnetic field. The rotor will follow this magnetic field causing the motor to turn. An AC drive converts the input frequency from 60Hz to between 0-300Hz and controls the speed of the motor proportionally.

This is achieved by supplying the drive from the power distribution system via a rectifier. The rectifier unit can be either unidirectional or bidirectional. When unidirectional, the AC drive can accelerate and run the motor by taking energy from the system. If bidirectional, the drive can also take the mechanical rotation energy from the motor, process it and then feed it back into the distribution system.

Simply put, an AC drive consists of a rectifier, DC circuit and inverter. The DC circuit will store the electrical energy from the rectifier for the inverter to use, and usually, it is stored in high power capacitors. A set of controls directs the rectifier and inverter to produce the desired AC frequency and voltage to control the motor speed.

In a standard configuration one converter in a group is assigned to one cargo pump in that group. This design limits the number of cargo pumps that can be run in parallel. Now consider a switchboard supplying 12 cargo pump motors divided into two groups of six controlled by three frequency controllers in each group. Three out of the six cargo pumps can be operated simultaneously in a group and six cargo pumps can run simultaneously in total. This system is designed to be flexible and cost effective as should one frequency controller fail another can be assigned to any cargo pump. MP